The present invention relates to encoding and decoding of information using materials that are capable of absorbing radiation over a wide range of infrared wavelengths and substantially non-absorbing in the visible wavelengths. Examples of such encoding of information are bar codes and area markings. Specifically, in the present invention, the information is encoded by depositing a material having near infrared absorption but no visible absorption on a common substrate. Methods and apparatus for retrieving the information use means for increasing the signal-to-noise ratio of the signal containing the information related to the dimensions of the deposited material or use properties of the absorption spectrum to deduce the dimensions of the deposited material.
Providing authenticity and security for photo identification and providing identifying information in consumer products are two of the most common applications of encoded markings such as bar codes. In many implementations, bar codes are black and white bars in which the information is encoded in the widths of the bars. The bar code is read with an optical reader comprised of a source that emits radiation in a range of wavelengths, means of scanning the radiation across the bar code and a detector that receives the reflected radiation. The information can be decoded from the electrical signal produced by the detector since the reflectance from the black bars is significantly different than that from the white bars.
While such an encoding method is commonly used, for a small object the bar code occupies a significant portion of the object and it detracts from the esthetics of the product. Also, a black and white bar code is susceptible to forgery once the code is deciphered or if the bar code adheres to a known standard.
In response to the first of these shortcomings, bar codes that are essentially non-visible and which can be read with infrared radiation have been developed. To implement the non-visible bar codes, the material deposited on the object or receiving medium must be invisible under radiation in the visible range but detectable under infrared radiation. Proposed materials included organic dyes such as cyanine based dyes and naphthoquinone dyes (U.S. Pat. No. 5,911,921) and inorganic materials such as Ytterbium phosphate (U.S. Pat. No. 5,911,921). The organic dyes are not stable in harsh environments and have some selective absorption in the visible range. The inorganic materials can be expensive to manufacture although the Ytterbium phosphate powder disclosed and claimed in U.S. Pat. No. 5,911,921 could represent a lower cost solution.
However, any bar code implemented with a material having high absorption in near infrared does not pose a solution to the problem of preventing forgeries since the use of an infrared scope or viewer will make the bar code detectable and the bar code could be counterfeited using infrared absorbers.
One solution, disclosed in U.S. Pat. No. 5,760,384, is to use a material that absorbs infrared radiation within a narrow band of wavelengths. One embodiment disclosed in U.S. Pat. No. 5,760,384 is a phthalocyanine which could exhibit instability in harsh environments.
An alternate approach to solving the problem of preventing forgeries is to detect the forgeries. U.S. Pat. No. 5,760,384 also discloses an apparatus and method for judging whether a bar code, comprising a material that absorbs infrared radiation within a narrow band of wavelengths, is real or a forgery. The disclosed method comprises detecting the reflectance at the peak absorption wavelength and at another neighboring wavelength away from the peak absorption wavelength.
U.S. Pat. No. 5,336,252 proposes another approach to preventing forgeries of infrared bar codes. In U.S. Pat. No. 5,336,252 the infrared bar code is covered with a layer of an ink that has high absorption in the visible and is transparent to the infrared. One embodiment is an ink prepared by mixing and dispersing a white pigment, such as titanium oxide or zinc oxide, with an extender such as calcium carbonate. While this approach will make it more difficult to forge the bar code, once the presence of the barcode is detected, it is not forgery proof.
All the above inventions relate to encoding the information in a marking created by depositing onto a medium a material that is capable of strongly absorbing radiation over a range of infrared wavelengths and substantially non-absorbing in the visible wavelengths. The necessity of strong absorption is derived from the decoding requirement that the reflectance, in the infrared range of wavelengths, from the infrared absorbing material is significantly different than that from the medium. This requirement will ensure that the electrical signal from the detector that receives the reflected infrared radiation is sufficiently distinct from the background noise so that it can be reliably decoded.
The large group of materials that are capable of mildly absorbing radiation over a range of infrared wavelengths and substantially non-absorbing in the visible wavelengths, such as most synthetic polymers (for example, polystyrene, Polyethylene terethalate), do not find application in the encoding of information as markings on a medium since the resulting electrical signal would not be sufficiently distinct from the background noise to be reliably decoded if read with the optical reader previously described. These large group of materials includes many low cost materials that would be attractive candidate materials if the information encoded in marks could be decoded.
The object of the present invention is to present methods for the use of lower cost, stable materials to encode information onto or on a base medium, and method and apparatus for reading the encoded information while retaining robustness to forgeries.
Information is encoded in markings on a base medium by depositing or intertexturing on the base medium a material where the surface dimensions, thickness and presence of the material contain the encoded information. In this invention, the material is capable of mildly absorbing radiation over a wide range of infrared wavelengths and substantially non-absorbing in the visible wavelengths. The encoding utilizes a lower cost, more stable material than a material that is capable of highly absorbing over a range of infrared wavelengths and substantially non-absorbing in the visible wavelengths. Inventive methods are then needed to ensure that the electrical signal from the detector that receives the infrared radiation reflected or transmitted from the medium and material disposed thereon is sufficiently distinct from the background noise so that it can be reliably decoded. Two different approaches for embodiments of such inventive methods are disclosed. In all embodiments disclosed, at least one of a plurality of sources of radiation is scanned over the medium and material disposed thereon, wherein said sources emit infrared radiation over a range of frequencies. Also in all embodiments disclosed, a portion of the radiation is reflected or transmitted from the medium and material disposed thereon, said reflected portion having a variable intensity over the scan. The reflected or transmitted portion is collected at a detector.
For one embodiment, the electrical signal from each of the at least one of a plurality of detectors is sampled at each of a plurality of spaced apart locations on the medium and material disposed thereon, the position of such locations being along the scan. At each of the plurality of spaced apart locations on the medium and material disposed thereon, the reflectance or transmittance at a plurality of select wavelengths, selected from the range of wavelengths emitted by the at least one of a plurality of sources, is determined. Knowledge of the reflectance or transmittance at a plurality of select wavelengths, which is the same as knowledge of the reflectance or transmittance spectrum at a plurality of select wavelengths, allows the use of spectrum identification methods, such as neural networks or principal component analysis, to identify the thickness and presence of the material at each of the plurality of spaced apart locations. Once the presence of the material at each of the plurality of spaced apart locations has been identified, the surface dimensions of the material can be deduced from the presence of the material at each of the plurality of spaced apart locations. From the thickness and surface dimension of the material, the encoded information can be decoded.
In another embodiment, the material disposed on the medium exhibits at least one relative absorption maximum in the infrared in addition to being capable of mildly absorbing radiation over a wide range of infrared wavelengths and substantially non-absorbing in the visible wavelengths. (The relative absorption maximum could be narrow or broad). The range of detectable wavelengths in the radiation reflected or transmitted from the medium and material deposited thereon is restricted. In one version of this embodiment, the restriction is selected to include one relative absorption maximum in the infrared. The restricted portion of the reflected or transmitted radiation is collected at a detector where it is generates an electrical signal, said signal having a variable intensity over the scan. The thickness and presence of the material are identified from the electrical signal. From the thickness and surface dimension of the material, the encoded information can be decoded.
In another version of the second embodiment, an oscillation is induced in the intensity of the restricted portion of the reflected or transmitted radiation collected at a detector, said oscillation having characteristics that vary along the scan. Inducing the oscillation in the collected radiation provides the opportunity to use known methods to increase the signal-to-noise ratio of the electrical signal corresponding to the collected radiation. After this step, the method is the same as the preceding version of the embodiment.
Apparatus providing means to perform the steps of each of the above described method are disclosed.